The present invention relates to a method and a system for detecting the presence, the absence or the variation in position of a metal body, and in particular is applicable to surveillance installations or centralized automatic systems.
In the field of inductive static sensors, devices are known at the present time which are essentially designed to increase the reliability of detection by combining a separate transmitter and detector in which, for example, the transmitter is used as the object to be detected or in which the transmitter and the receiver are coupled by a passive transmission system or, again, in which the object to be detected comes and disturbs the electromagnetic coupling between the transmitter and the receiver. In each of these examples the inductive proximity sensor comprises two separate devices one of which carries out the transmission function and the other the detection furnction. In these examples the transmitter is either not modulated or modulated in a fixed manner independent of the electromagnetic condition of its surroundings. Furthermore, the data indicating the presence or absence of the object being detected depends on the amplitude of the signal received by the receiver-detector, with the permanent fixed modulation only being there to pick this out.
In addition surveillance installations or centralized automatic systems are known in which the data they require are sent by wired connections, either with loop or point-to-point structures. The wired connections connecting the various sections of these known conventional systems have the advantage of being simple, with known and reliable technology. However, the installation cost of these links is often much greater than the total cost of the equipment applied for detecting and processing the data, which finally makes wired systems very expensive. This high cost is a disadvantage in the field of protection against intrusions into low risk premises like offices, shops and private dwellings in which the overall cost of the installation is one of the basic factors.